dam

The Hoover Dam is 726 feet (221 meters) high. It is on the Colorado River between Nevada and Robert CameronStone/Getty Images

A dam is a barrier that blocks flowing water or sends it in a new direction. People and animals build dams to control rivers and to create ponds or lakes. A beaver's dam works in much the same way as the giant dams that people build. Both must be high enough and strong enough to keep the water from flowing forward.

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